1. Field of the Invention
The present invention relates to a gas turbine and, specifically, the present invention relates to a cooling system for a gas turbine combustor.
2. Description of the Related Art
In general, there are two types of cooling systems for the wall of a gas turbine combustor. One is a compound air cooling system employing both convective cooling and film air cooling using air as cooling fluid and another is steam cooling system using steam as cooling fluid. In an actual gas turbine, one of the compound air cooling system and the steam cooling system is selected according to the temperature of combustion gas at the inlet of turbine.
FIG. 1 is a sectional view showing a general construction of a gas turbine combustor employing a compound air cooling system.
In FIG. 1, reference numeral 1 designates a combustor of a gas turbine as a whole. The combustor 1 consists of a combustion tube 5b which acts as a combustion chamber for burning fuel injected from fuel nozzles 33 and a tail pipe 5a which directs combustion gas generated in the combustion tube to the first stage stator of the turbine. The combustor tube 5b and the tail pipe 5a are made as separate parts and joined together to form a combustor 1.
Fuel is injected into the combustion tube 5b from the main nozzles 33 as a premixed air-fuel mixture. The air-fuel mixture is ignited by a pilot flame formed by a pilot nozzle 31 and generates a premixed flame in the combustion tube.
FIG. 8 is a enlarged section of the wall of the combustor tube 5b employing a conventional compound air cooling system. As can be seen from FIG. 8, in an actual gas turbine, the combustor tube 5b is formed by joining a plurality of cylindrical shells 55 having different diameters. The respective shells 55 are aligned in the axial direction and are joined to each other through stepped diameter portions thereof. Each of the shells 55 acts as a structural member forming the combustor tube 5b. A heat insulating member 155 is disposed at inside of each cylindrical shell 55 in order to protect the shell from the flame in the combustor tube and, thereby, preventing a strength degradation of the shell as a structural member.
In the conventional cooling system, fin-rings are used for the heat insulating members 155. The fin-ring consists of a cylindrical member having numerous grooves on the outer surface thereof extending in the axial direction. Each of the fin-rings 155 is held inside of the shell by attaching one end thereof to the smaller diameter portion of the corresponding shell 55 (i.e., a fuel nozzle side end of the shell 55), for example, by brazing.
In this system, pressurized air in the casing 7 (FIG. 1) is introduced from inlet openings 57 distributed around the smaller diameter portion of the shell 55 into the space between the shell 55 and the fin-ring 155. Air introduced into the space passes through the axial grooves outside of the fin-ring 155 and cools fin-ring 155 by convective cooling. After passing through the axial grooves, air is injected from the outlet 159 at the end of the fin-ring 155 in the direction along the inner surface of the heat insulating member (in FIG. 8, indicated by reference numeral 155b) adjacent thereto. Thus, the wall surface of the combustion chamber, i.e., the inner surface of the adjacent fin-ring 155b is cooled by the film of the injected air.
On the other hand, FIG. 10 is a sectional view similar to FIG. 1 showing a gas turbine combustor employing a conventional steam convective cooling system.
Since the heat-transfer coefficient of air is relatively low, sufficient cooling can not be obtained by convective cooling and, usually, a compound air cooling system using both convective cooling and film air cooling is employed in the air cooling system. However, compound air cooling system has its inherent problem. In the compound air system, air used for film air cooling is injected into the combustion tube and mixes with combustion gas. This cause dilution of combustion gas and lowers its turbine inlet temperature and, thereby, causes deterioration in the gas turbine output and efficiency.
In order to prevent this problem, the combustor in FIG. 10 employs steam cooling system using steam convective cooling instead of compound air cooling. Since the heat-transfer coefficient of steam is larger than that of air, the combustor is sufficiently cooled solely by convective cooling in the steam cooling system.
In FIG. 10, reference numerals the same as those in FIG. 1 denotes elements similar to those in FIG. 1.
The combustor in FIG. 10 is a one-piece construction in which the combustion tube 5b and the tail pipe 5a are formed as an integral part. Therefore, the combustion tube 5 in the combustor 1 in FIG. 10 has outlet 52 at one end thereof in order to supply combustion gas to the first stage stator of the turbine.
The combustion tube 5 in FIG. 10 has a double-wall construction including an outer shell (outer wall) and an inner shell (inner wall). The space between the outer shell and inner shell acts as a passage for cooling steam. Cooling steam is supplied to the cooling steam passage between the outer and the inner shells from a steam inlet connection 507 disposed near the center of the length of the combustion tube 5. The steam introduced into the cooling passage is divided into two streams flowing in the directions opposite to each other. Namely, a portion of the cooling steam flows through an upstream cooling passage in the wall of the combustion tube 5 from the inlet 507 in the upstream direction (i.e., towards the main nozzle 33 side) and other portion of the cooling steam flows through a downstream cooling passage in the wall of the combustion tube 5 from the inlet 507 in the downstream direction (i.e., towards the outlet 52 of the combustion tube). Cooling steam outlet pipes 509a and 509b are connected to the cooling steam passage at the upstream (main nozzle 33 side) end and the downstream (outlet 52 side) end of the combustion tube 5, respectively, in order to collect cooling steam after it cooled the combustor walls. Since the heat-transfer coefficient of steam is-relatively large, the walls of the combustor are sufficiently cooled by convective cooling using cooling steam.
The conventional compound air cooling system and the steam cooling system as explained above include respective drawbacks.
In the first place, in the compound air cooling system using the fin-rings, consumption of cooling air is large.
FIG. 9 is a cross sectional view taken along the line IXxe2x80x94IX in FIG. 8. As explained before, the fin-ring 155 is provided with grooves extending along the axial direction on the outer surface thereof. When the fin-ring 155 is attached to the shell 55, an annular clearance 155c must be disposed between the shell 55 and fin-ring 155 in order to avoid contact between the shell 55 and fin-ring due to thermal expansion of the fin-ring. When the manufacturing tolerance and the tolerance in the assembling of the combustor are taken into account, the required width of the clearance 155c becomes almost the same as the depth of the grooves 155b in some cases. Therefore, in the conventional compound air cooling system, since a relatively large clearance 155c between the outer surface of the fin-ring 155b and the inner surface of the shell 55, a large amount of cooling air passes through the clearance 155c in the axial direction and flows into the combustion chamber without passing through the grooves 155b. In other words, a large portion of the cooling air introduced from the inlet 57 flows into the combustion chamber without being used for cooling the fin-ring 155. Consequently, in order to obtain sufficient convective cooling of the fin-ring 155, the amount of cooling air supplied from the inlet 57 must be increased so that a sufficient amount of air passes through the grooves 155b. 
Further, a large amount of cooling air which passes through the annular clearance 155c and does not contribute to convective cooling of the fin-ring 155 also flows into the combustion chamber and dilutes the combustion gas. Therefore, the drop of the combustion gas temperature due to introduction of cooling air becomes large in the conventional compound air cooling system.
The width of the annular clearance 155c may be reduced if the tolerances of machining and assembly of the shell 55 and fin-ring 155 are smaller. However, smaller tolerance in machining and assembly of these parts causes an increase in the cost and time required for manufacturing the combustion tube 5.
Further, in the conventional compound air cooling system using the fin-ring 155, since the fin-ring 155 is attached to the shell 55 at only one end thereof, it is difficult to increase the structural strength of the combustion tube assembly 5.
On the other hand, the problems such as those in the conventional compound air cooling system as explained above does not occur in the steam cooling system in FIG. 10. However the steam cooling system also has an inherent problem of a large consumption of cooling steam.
When the steam cooling system is used, as explained in FIG. 10, cooling steam is introduced into the cooling passage of the combustion tube 5 from the steam inlet 507 disposed near the center of the length of the combustion tube 5 and passes through the upstream cooling passage and the downstream cooling passage in directions opposite to each other. Usually, cooling steam is supplied to both an upstream and a downstream cooling passage at a same supply conditions.
However, although cooling steam is supplied at the same supply conditions, the heat loads on the upstream cooling passage and the downstream cooling passage are not the same and, usually, the heat load on the downstream cooling passage is larger than that on the upstream cooling passage.
Air-fuel mixture is injected into the combustion tube 5 from the main nozzles 33 at the upstream end thereof and it burns while it flows towards the outlet 52 of the combustion tube 5. Therefore, since the combustion of air-fuel mixture is not completed in the upstream half of the combustion tube 5, the temperature of the combustion gas is relatively low in the upstream half of the combustion tube 5. On the other hand, since combustion of air-fuel mixture is completed at the downstream half of the combustion tube 5, the temperature of combustion gas is higher at the downstream half of the combustion tube 5 than at the upstream half thereof. Consequently, heat load on the downstream cooling passage becomes higher than that on the upstream cooling passage.
However, since the supply conditions of the cooling steam to both cooling passage are the same, the supply conditions of cooling steam must be adjusted to meet the requirement of the downstream cooling passage where the heat load becomes the maximum in the conventional steam cooling system. This means that the wall of the upper half of the combustion tube 5 is cooled more than necessary (i.e., excessive cooling occurs at the upper half of the combustion tube 5). Therefore, in the conventional steam cooling system, an excess amount of cooling steam is required to cool the upper half of the combustion tube 5 excessively.
In view of the problems in the related art as set forth above, the object of the present invention is to provide a cooling system for a gas turbine combustor which is capable of reducing consumption of cooling air and/or cooling steam without lowering the cooling capacity.
The objects as set forth above are achieved by a cooling system for a gas turbine combustor, according to the present invention, comprising a combustion tube having a cylindrical shell which forms a combustion chamber therein for burning fuel, a cylindrical heat insulating member disposed in the shell and forming a wall of the combustion chamber, the heat insulating member being provided with a plurality of cooling air passages extending therein in the axial direction of the combustion tube for introducing cooling air into the cooling air passages from cooling air inlets of the respective cooling air passage disposed at one end of the heat insulating member and discharging cooling air, after cooling air passes through the cooling air passages, from cooling air outlets disposed at the other end of the heat insulating member in the axial direction along the inner surface of the heat insulating member, wherein, the heat insulating member is attached to the shell by joining one end thereof to the inner surface of the shell and provided with a sealing means on the outer surface of the heat insulating member at the portion between the cooling air inlets and the cooling air outlets for preventing cooling air from flowing into the combustion chamber through a clearance between the outer surface of the heat insulating member and the inner surface of the shell.
According to the present invention, since the cooling air passage is formed within the heat insulating member. Therefore, different from grooves in the related art, the cross section of the cooling air passage has no open side (i.e., the respective cooling air passages are surrounded by walls on all sides thereof). Further, since the seal ring disposed between the shell and the heat insulating member blocks cooling air passing through the annular space between the heat insulating member and the shell. Therefore, all of cooling air supplied to the heat insulating member passes through the cooling air passage and contributes to convective cooling of the heat insulating member. Thus, according to the present invention, the amount of cooling air requited for cooling the heat insulating member can be substantially reduced compared with that required when the fin-ring is used as the heat insulating member.
According to another aspect of the present invention, there is provided a cooling system for a combustor of a gas turbine which generates combustion gas by burning fuel for driving a turbine comprising a cylindrical combustion tube having an inlet end and an outlet end and forming a combustion chamber therein for burning fuel supplied from the inlet end thereof and supplying combustion gas to a turbine from the outlet end thereof, a plurality of cooling steam passages formed in the wall of the combustion tube and extending along the length of the combustion tube between the inlet end and the outlet end, the cooling steam passages including first cooling steam passages in which cooling steam flows in a first direction and second cooling steam passages in which cooling steam flows in a second direction opposite to the first direction.
According this aspect of the invention, two groups of cooling steam passages, i.e., the first cooling steam passages and the second cooling steam passages are provided in the wall of the combustion tube. In the first and the second cooling steam passages, cooling steam flows in the direction opposite to each other. Therefore, the average of the temperatures of cooling steam flowing through both first and second cooling steam passages become uniform along the length of the combustion tube and excessive cooling of the inlet side end of the combustion tube does not occur. Thus, the combustion tube is suitably cooled with smaller amount of cooling steam and the amount of cooling steam required for cooling the combustion tube can be reduced.
According to another aspect of the present invention, there is provided a cooling system for a combustor of a gas turbine which generates combustion gas by burning fuel for driving a turbine comprising a cylindrical combustion tube having an inlet end and an outlet end and forming a combustion chamber therein for burning fuel supplied from the inlet end thereof and supplying combustion gas to a turbine from the outlet end thereof, a cooling air passage disposed on the outer surface of the combustion tube for guiding cooling air along the outer surface of the combustion tube from a cooling air inlet thereof disposed at the portion near the center of length of the combustion tube to the cooling air outlet thereof disposed at the portion near the inlet end of the combustion tube and supplying cooling air from the cooling air outlet to the combustion chamber from the inlet end of the combustor so that cooling air after passing through the cooling air passage is used for burning fuel in the combustion chamber, and
a cooling steam passage formed in the wall of the combustion tube and extending from a first portion near the center of the length of the combustion tube and a second portion near the outlet end of the combustion tube, the cooling steam passages introduce cooling steam thereinto from a cooling steam inlet disposed at one of the first and second portion and guiding cooling steam within the wall of the combustion tube in the direction along the length thereof to a cooling steam outlet disposed at the other of the first and second portion.
According to this aspect of the present invention, a downstream half of the combustion tube, where the combustion gas temperature is relatively high, is cooled by a cooling steam having a heat-transfer coefficient higher than cooling air, and a upstream half of the combustion tube, where the combustion gas temperature is relatively low is cooled by cooling air. Therefore, consumption of cooling steam is reduced compared with the case where both downstream half and upstream half of combustion tube are cooled by cooling steam. Further, cooling air after cooling the upstream half of the combustion tube is used as combustion air in this aspect. of the invention. Therefore, cooling air after cooling the combustion tube is used for burning fuel and does not dilute combustion gas. Thus, the temperature drop of combustion gas due to dilution, as well as a shortage of combustion air, does not occur.